1. Field of the Invention
The present invention relates to a method of inspecting a heat exchanger comprising a plurality of tubes, and in particular to a nondestructive method of inspecting heat exchanger tubes that employs adaptive thresholding based on noise.
2. Related Art
Heat exchangers, such as, for example, steam generators used in pressurized water nuclear powered electric generating systems, generally include thousands of U-shaped heat exchanger tubes disposed within a generally cylindrical pressure vessel. The ends of the heat exchanger tubes are secured within a transverse plate called a tubesheet, which separates the steam generator into a primary side and a secondary side. Heated primary fluid from the nuclear reactor is passed through the tubes to effectuate a heat transfer with the secondary working fluid which, in turn, drives the turbo-machinery used to generate electricity. The primary fluid can be radioactive. Accordingly, to prevent leakage of the reactor coolant into the secondary side of the generator, which would contaminate the steam, the heat transfer tubes must be periodically inspected for flaws and degradation such as cracks, pits, dents and tube wall thinning. If a degraded tube is discovered, it is typically plugged at both ends. In view of the thousands of tubes in the steam generator, plugging of a few tubes does not appreciably affect the efficiency of the heat transfer.
Eddy current testing is a well known, commonly used method of nondestructive testing of steam generator tubes. Generally, in performing an eddy current test on steam generator tubes, a sensor or probe is advanced through the tube as signals are generated and recorded for later analysis. See, e.g., U.S. Pat. No. 3,302,105 (illustrating and describing the eddy current signatures of various types of tube defects); see also U.S. Pat. Nos. 3,693,075; 4,194,149; 4,207,520; and 4,631,688. U.S. Pat. No. 4,763,274, which was filed on Jun. 24, 1986 and issued to the assignee hereof, discloses eddy current inspection processes for nuclear steam generator tubes and computer analysis of the eddy current data for automatically detecting flaws in the heat transfer tubes of a steam generator.
Automatic analysis systems employ what is commonly known as flaw categorization, which is a process wherein the eddy current data that is collected is analyzed flaws in the tube are identified and categorized based on a set of logic based rules. The logic based rules typically employ a set of minimum thresholds that are defined by an analyst. A low fixed threshold may create a large number of false positive reports on noisy tubes, and a high fixed threshold may lead to certain flaw signals not being addressed or properly categorized.
Thus, there is a need for an improved nondestructive method of inspecting heat exchanger tubes that addresses the problems associated with minimum thresholds described above.